The hypothesis that transmission of nociceptive information from spinal cord to brain is dependent on both impulse frequency and extensive central spatial recruitment of nociceptive neurons is supported by neurological, anatomical, and physiological evidence. Since all of these lines of evidence are indirect, an experimental approach is needed that examines the effects of graded nociceptive stimuli on the spatial distribution of spinal cord neuronal responses. The present proposal utilizes two conceptually related and methodologically different ways of mapping the rostral-caudal, dorsal-ventral, and mediolateral spread of spinal cord neuronal responses to graded nociceptive and non-nociceptive stimuli (43-51 degrees C) and to non-noxious mechanical stimuli applied to the cat's hind limb. The (C14)-2- deoxyglucose method will examine the spatial distribution of increased metabolic activity within spinal cord grey matter laminae I-VIII, and electrophysiological single neuron recording will map impulse frequency responses of nociceptive neurons at rostral-caudal levels. Results from both methods will be compared. The electrophysiological method will be of critical importance in identifying which physiological types of spinal cord sensory projection neurons and interneurons are involved in spatial recruitment. Antidromic activation from ascending spinal tracts will be of critical importance in this identification. The results of this proposal should provide a most direct and extensive characterization of the role of spatial and temporal factors in spinal cord encoding of nociceptive information. The spatial recruitment mechanisms elucidated in this study may help explain chronic clinical pain mechanisms and the mechanism of return of pain following surgical lesions of the peripheral or central nervous system.